1. Field of the Invention
The present invention relates to an apparatus for cleaning up reactor coolant in a boiling water reactor plant and, more particularly, to a reactor-coolant cleanup apparatus suitable for use in mitigating radiation exposure which may occur during scheduled inspections due to the presence of a piping provided for taking out reactor coolant at the bottom of a reactor pressure vessel, as well as to a method of controlling such an apparatus.
2. Description of the Related Art
As disclosed in Japanese Patent Unexamined Publication No. 58-201094, a conventional type of reactor-coolant cleanup system for use in a boiling water reactor plant is in general arranged such that reactor coolant is extracted from its reactor pressure vessel through a piping connected to the lowest portion of the reactor pressure vessel and a piping branching off a primary loop recirculation piping. The reactor coolant flows in both of these pipings intermingle with each other within a primary containment vessel and is, in turn, passed through a heat exchanger, a pump, and a cleanup device which constitutes part of the reactor-coolant cleanup system. Thereafter, this reactor coolant intermingles with the flow of water fed through a reactor feedwater piping, and is returned to the reactor pressure vessel.
The piping disposed at the lowest portion of the reactor pressure vessel performs the following functions. The first function is to discharge the crud component accumulated at the bottom portion of the reactor pressure vessel from the reactor pressure vessel together with the reactor coolant. The second function is to completely discharge the reactor coolant from the reaction pressure vessel for the purposes of inspection or modification. The third function is to circulate low-temperature reactor coolant stagnating at a lower portion of the reactor pressure vessel by means of the reactor-coolant cleanup system (without utilizing any recirculation system) when the reactor is in a hot stand-by state in which the normal running of the reactor is not carried out.
The piping connected to the lowest portion of the reactor pressure vessel is arranged to discharge the reactor coolant from the inside of a core shroud surrounding a reactor core directly into the exterior of the reactor pressure vessel. For this reason, if an accident such as breakage should occur in such a piping, the reactor coolant is fed into the reactor core through an emergency core cooling system. However, even after the reactor core has been flooded, the discharge of the reactor coolant through the piping is continued. Accordingly, it is impossible to form the piping from a pipe having a sufficiently large diameter, and the diameter of this piping is commonly one-fourth to one-fifth the diameter of a piping provided for taking out water in the recirculation piping. As a result, the crud component contained in the reactor coolant tends to easily stick to the inner surface of the piping and there is a tendency for the dose rate of the piping to increase.
The outlet piping and the inlet piping of the primary loop recirculation piping are connected to the reactor pressure vessel in the outside of the reactor core shroud (not shown). Accordingly, even in a case where the primary loop recirculation piping is partially broken, after the reactor core has been flooded by the operation of the emergency core cooling system, the reactor coolant in the reactor core is not discharged through the recirculation piping directly into the exterior of the reactor pressure vessel. It is, therefore, possible to increase the diameter of the recirculation piping in order to prevent the crud component from sticking to the inner surface of the recirculation piping.
Further, the piping connected to the lowest portion of the reactor pressure vessel is located at a structurally lower position. It follows, therefore, that the proportion of the length of horizontally extending pipe portions to the overall length of such a piping is large. This fact also causes the crud component contained in the reactor coolant to stick to the inner surface of the piping and, hence, increases the dose rate of the piping.
As described above, in the prior art, the crud component in the reactor coolant sticks to the inner surface of the piping connected to the lowest portion of the reactor pressure vessel to cause an increase in the dose rate of the piping. This increase in the dose rate constitutes a radiation source which brings about an increase in the dose rate of the surroundings within the primary containment vessel during scheduled inspections. In the prior art, however, no consideration is given to this problem, and it has been impossible, therefore, to avoid the problem that workers who need to enter the primary containment vessel for the purposes of scheduled inspections may undergo serious radiation exposure. In order to mitigate this radiation exposure, it has been proposed that a shielding made of lead, iron, or the like be employed. Although this proposal utilizes a method of installing a lead plate or an iron plate directly onto the piping, it is necessary to install a support for preventing the weight of the shielding from being applied directly to the piping, and the installation of this kind of support incurs an increase in cost. Moreover, the installation of this support inevitably narrows the working space in the primary containment vessel and, therefore, leads to a deterioration in the efficiency of operation during scheduled inspections.